Arrays of photosensitive diodes are used in various applications including radiation detectors, imagers and optical position encoders. The arrays may be formed as one- or two-dimensional arrays of aligned photodiodes, or, for optical shaft encoders, a circular or semicircular arrangement of diodes.
A significant and well recognized problem associated with photodiode arrays is "cross-talk" occurring because of minority carrier leakage current between lit and unlit diodes. Such cross-talk can be particularly troublesome in photodetector arrays for optical encoders. Various approaches have been used to minimize such cross-talk, including providing inactive photodiodes to balance the leakage current, as described in U.S. Pat. Nos. 4,904,861 and 4,998,013 to Epstein, et al.
The problem of cross-talk between diodes in arrays for optical encoders becomes even more acute as the size of the encoder arrays and the relative size and spacing of the diodes is reduced. For micromechanical applications, it would be desirable to produce optical shaft position encoders having lateral dimensions of a few millimeters or even less. To obtain the desired precision angle measurements from the output of an encoder in this size range, the diodes in the arrays must be small and spaced closely together, e.g., with typical diode widths in the range of a few microns or tens of microns and spacing between adjacent diodes of a few microns. At the typical doping levels utilized for the base layer of semiconductor in diode arrays of this type, the diffusion length of minority carriers generated by photon interaction in the semiconductor is in the range of at least many tens of microns, and such minority carriers have the potential to affect signals at diodes several hundreds of microns away from the region at which the minority carriers were generated. Consequently, as the size of the photodiode arrays is reduced, the cross-talk problem becomes much more severe.
The use of trenches, moats or insulating structures between photodiodes or other active devices to provide isolation between the devices is known. See, e.g., the U.S. Pat. Nos. 4,639,756 to Rosbeck, et al., 4,868,622 to Shigenaka, 4,868,623 to Nishiura, 5,049,962 to Huang, et al., 5,061,652 to Bendernagel, et al., 5,391,236 to Krut, et al., 5,430,321 to Effelsberg, and 5,438,217 and 5,500,376 to Ishaque, et al. However, such prior approaches are typically not well suited to forming closely spaced miniaturized diode arrays, wherein the spacing between diodes should be in the range of a few microns, or require complex processing steps including passivation of p-n junctions exposed by the trenches between the active devices.